This invention relates generally to jewelry composed of metallic components and methods of manufacturing such components, and in particular relates to jewelry, such as necklaces, bracelets, earrings, gem stone settings, and the like, which comprise a metallic component consisting of a shape memory metal alloy in the form of a wire, rod or cable, and in particular a shape memory metal alloy composed of a near stoichiometric alloy of nickel and titanium, such as the alloy known as Nitinol.
A shape memory alloy is an alloy which upon being thermally treated at a very high temperature (typically at least about 500 degrees C. or above) becomes fixed in a given shape due to crystalline alignment, yet when cooled below a transition temperature is relatively easily deformable. Often the alloy is shaped on mandrels or forms to create the desired memory shape. Cooling after forming is preferably performed in a rapid manner through a water quench or rapid air cool. The particular temperatures and treatment times are dependent on the particular alloy being treated and the shape and thickness of the alloy. Superelastic characteristics are inherent in the alloys after particular treatments. Alloys subjected to extreme plastic deformation, such that the wire or rod is bent or kinked, can be returned to the pre-deformation configuration upon reheating to a temperature above the transition temperature, the alloy automatically recovers its pre-deformation fixed shape due to realignment of the crystalline phase.
Nitinol is the name given to a family of intermetallic alloys of nickel and titanium which show unique properties of shape memory and superelasticity. These properties were discovered in near equiatomic Ni—ti alloys at the Naval Ordinance Laboratory. Nitinol comprises from about 50 to 60 percent Ni and about 40 to 50 percent Ti, with less than about 5 percent other elements. A very common alloy is Nitinol-55, which contains approximately 55 percent Ni. The temperature for the shape memory reaction can be varied from below zero degrees C. to about 100 degrees C. by changing the Ni content of the alloy.
Shape memory is physical phenomenon by which a plastically deformed metal is restored to its original shape by a solid state phase change caused by heating. The explanation of the shape memory response is found in the strong crystallographic relationship between the phase stable at low temperature, called martensite (a close packed monoclinic crystalline structure), and the phase stable at high temperature, called austenite (an ordered body centered cubic phase crystalline structure). A wire or rod to be formed into a particular memory shape is formed, usually on a mandrel or other fixture, and heated to a temperature through the austenite start temperature, A(s), and above the austenite finish temperature, A(f), and held for a suitable time period at temperature. The wire is then rapidly cooled, through the martensite start temperature, M(s), and below the martensite finish temperature, M(f). The stress due to the constrained shape produces twins in the martensite phase which are reversible realignments of the crystal lattice. This phase structure is easily deformed into other shapes by continued realignment and preferential growth of favorably oriented twins. To recover the imparted memory shape, the wire is reheated to above A(f), which reverses the alignment of the twins and reforms the austenite crystalline structure, the wire automatically resuming the memory shape.
Superelasticity, reversible non-linear elastic deformation, is imparted to Nitinol by a particular treatment during the austenite phase. The Nitinol wire is stained, cold-worked, drawn, formed or the like, at a temperature above A(s) but below the maximum temperature at which superelasticity is obtained.
Typical representative properties of Nitinol alloys, which will vary somewhat dependent on composition, are as follows:
Melting point1310 degrees C.Density  6.5 gm/ccYoung's modulus 120 Gpa (austenite) 50 Gpa (martensite)Yield strength 379 Mpa (austenite) 138 Mpa (martensite)Ultimate tensile strength690 to 1380 MpaElongationup to 20% or moreShape memorytransformation temp.−50 to 100 Degrees C.recoverable strain6.5 to 8.5%superelastic recoverable strainup to 8%transformation fatigue lifeseveral hundred cycles at 6% strain10,000 cycles at 2% strain1,000,000 cycles at 0.5% strain
The following examples illustrate the different characteristics which can be imparted to Nitinol alloys by varying composition and treatment.
Alloy #1 (superelastic): 55.8±0.5 wt % Ni, balance of Ti, α 0.5 C, O, Fe                A(s)=−10 degrees C.±5        A(f)=+5 degrees C.±5        
typical tensile properties of cold-drawn and tempered material:                Upper superelastic plateau stress: 55 ksi        Lower superelastic plateau stress: 20 ksi        Permanent set after 6% strain: 0.1%        Yield strength of the martensite after transition: 118 ksi        Ultimate tensile strength: 155 ksi        Elongation to failure: 17.5%        Maximum strain recovery: 8%        
Alloy #2 (high-strength, superelastic):                55.9±0.5 wt % Ni, balance of Ti, α 0.5 C, O, Fe        A(s)=−20 degrees C.±5        A(f)=−5 degrees C.±5        
typical tensile properties of cold-drawn and tempered material:                Upper superelastic plateau stress: 70 ksi        Lower superelastic plateau stress: 30 ksi        Permanent set after 6% strain: 0.1%        Yield strength of the martensite after transition: 185 ksi        Ultimate tensile strength: 210 ksi        Elongation to failure: 12.5%        Maximum strain recovery: 8%        
Alloy #3 (high-temperature, shape memory):                55.5±0.5 wt % Ni, balance of Ti, α 0.05 C, O, Fe,                    α 0.005 H, α 0.01 other trace elements                        A(s)=95 degrees C.±5        A(f)=115 degrees C.±5        
typical tensile properties of cold-drawn and tempered material:                Ultimate tensile strength: 180 ksi        Elongation to failure: 12% min.        Maximum strain recovery: 8%        
Alloy #4 (body-temperature):                55.5±0.5 wt % Ni, balance of Ti, α 0.05 C, O, Fe,                    α 0.005 H, α 0.01 other trace elements                        A(s)=15 degrees C.±5        A(f)=35 degrees C.±5        
typical tensile properties of cold-drawn and tempered material at 36-38 degrees C.:                Upper superelastic plateau stress: 50 ksi        Lower superelastic plateau stress: 2 ksi        Permanent set after 6% strain: 0.5%        Ultimate tensile strength: 180 ksi        Elongation to failure: 12% min.        Maximum strain recovery: 8%        
Alloy #5 (chrome-doped, superelastic):                55.8±0.5 wt % Ni, balance of Ti, 0.2-0.3 Cr,        α 0.05 C, O, Fe, α 0.005 H, α 0.01 other trace elements        A(s)=−30 degrees C.±5        A(f)=−10 degrees C.±5        
typical tensile properties of cold-drawn and tempered material:                Upper superelastic plateau stress: 80 ksi        Lower superelastic plateau stress: 35 ksi                    Permanent set after 6% strain: 0.1%                        Ultimate tensile strength: 225 ksi        Elongation to failure: 10% min.        Maximum strain recovery: 8%        
Alloy #6 (high-strength, superelastic, Ni—Ti—Fe):                53.5±1.0 wt % Ni, balance of Ti, 1.0-2.0 Fe,        α 0.05 C, O, α 0.005 H, α 0.01 other trace elements        A(s)=−30 degrees C.±5        A(f)=−10 degrees C.±5        
typical tensile properties of cold-drawn and tempered material:                Upper superelastic plateau stress: 100 ksi        Lower superelastic plateau stress: 65 ksi        Permanent set after 6% strain: 0.5%        Ultimate tensile strength: 210 ksi        Elongation to failure: 10%        Maximum strain recovery: 8%        
In comparison to other metals typically used in the manufacture of jewelry, and in particular in comparison to metals having structural properties considered as good, such as for example stainless steel, Nitinol exhibits dramatically increased tensile strength, at least an order of magnitude greater than stainless, and greater flexibility, at least 50 percent greater than stainless, and much greater superelasticity characteristics. Individual wire strands or thin rods may be intertwined to create bundles of strands or cables, with varied pitch, helix angle, diameters, etc.
Nitinol may also exhibit pseudoelastic properties, which occurs in a relatively narrow temperature range slightly above the stress-free austenite to martensite transformation temperature, and involves the creation of stress-induced martensite which simultaneously undergoes strain as it forms to relieve the applied stress. When the applied stress is removed, the thermally unstable martensite reverts to austenite, and the strain spontaneously returns to zero. This behavior gives a very high apparent elasticity to the material without inducing any permanent strain, but is limited in the temperature range where it can be utilized in a given alloy. Outside of the temperature range, heat will be required to resume the memory shape after deformation.
Objects of this invention are to provide for jewelry which is composed in whole or in part of a shape memory alloy, and in particular of Nitinol, such that the mechanical memory characteristics and/or superelasticity characteristics of the alloy are utilized to improve the ease of manufacture, to enable jewelry to be manufactured in ways which cannot be accomplished by non-shape memory alloys, to provide new manufacturing methods utilizing shape memory alloys, to provide cast pieces of jewelry, jewelry containing shape memory alloy members, wires or braided wire as support members and/or aesthetic elements, and jewelry composed of braided wires to form cable pieces, wherein the superelasticity and shape memory characteristics improve the durability, functionality and ease of repair of the jewelry pieces.